For memory devices and for numerous other applications, bi-stable devices or circuits are used. For example, for storing one bit of information in a memory, a bi-stable device can be used which is switchable between (at least) two different and persistent states. When writing a logical “1” into the device, it is driven into one of the two persistent states and when writing a logical “0”, or erasing the logical “1”, the device is driven into the other of the two different states. Each of the states persists until a next step of writing information into the device or erasing information in the device proceeds. A huge number of such bi-stable devices arranged in one or more arrays may form an EEPROM (EEPROM stands for electrically erasable programmable read-only memory) as a separate memory device or as part of an even more complex device.
An example for such a bi-stable device is a resistor with two (or more) reversibly switchable and persistent resistance-states. The resistor is made of a material with respective reversibly switchable and persistent conductivity states. The conductivity states of the resistor are changed by applying short voltage or current pulses to the resistor. These pulses should be larger than a given threshold VT. The conductivity state of the resistor can be read or analyzed by applying other pulses which are non-destructive to the conductivity state if they are much smaller than VT.
U.S. Pat. No. 6,204,139 describes a method for switching properties of perovskite materials used in thin film resistors. The properties, in particular the conductivity, are switched reversibly by short electrical pulses. Application of the method for non volatile memory units and for sensors with changeable sensitivity is proposed.
U.S. Pat. No. 6,531,371 describes an electrically programmable resistance cross point memory. At cross points of bit lines and word lines, perovskite material acts as variable resistors the resistance values of which can be changed reversibly and with hysteresis.
U.S. 20030156445 A1 describes a method of changing the resistance of a perovskite metal oxide thin film device by means of a resistance change producing pulse.
The articles “Reproducible switching effect in thin oxide films for memory applications” (A. Beck et al., Applied Physics letters, Vol. 77, No. 1, July 2000) and “Current-driven insulator conductor transition and non-volatile memory in chromium-doped SrTiO.sub.3 single crystals” (Y. Watanabe et al., Applied Physics Letters, Vol. 78, No. 23, June 2001) and the international application publication WO 00/49659 A1 describe materials and classes of materials with hysteretically switchable electrostatic resistance, and simple resistor devices made from these materials.
The article “Electrical current distribution across a metal-insulator-metal structure during bistable switching” (C. Rossel et al., Journal of applied Physics, Vol. 90, No. 6, September 2001) provides evidence that bi-stable switching is localized to filaments.
In thin films, clear evidence was given that one or several current filaments can occur. The high or low resistance state of the thin film or a memory cell comprising the thin film is correlated with the number of filaments and with the intensity of each filament.
The filaments do not exist in recently fabricated materials. Rather, after fabrication of a device, one or several filaments are generated in a forming process before the device or, to be more specific, the filament, can be switched between two or more resistant conductivity states.
The forming process is voltage controlled. Its success or duration depends on several parameters of the insulator or dielectric material (stoichiometry, doping, and thickness) and of the electrodes and on ambient conditions such as the temperature. In a large number of devices, for example in a large number of storage cells in an integrated memory device, the number, intensity and position of the filaments in each cell are more or less randomly distributed. As a consequence, the electronic and electrical properties of the single storage cells are randomly distributed, as well. The time-consuming and hardly reproducible forming process and the variability of the properties of the memory cells are severe drawbacks of conventional resistors with bi-stable switchable resistance and of devices comprising such a resistor.